Television engineers have given considerable thought to ghost-cancelation circuitry for inclusion in television receivers that also include a display device for reproducing the television image in a form suitable for viewing by humans. Ghost images, caused by multipath reception and commonly referred to as "ghosts", are a common occurrence in television pictures that have been broadcast over the air or have been transmitted by cable.
The signal to which the television receiver synchronizes is the strongest of the signals it receives, which is called the reference signal, and is usually the direct signal received over the shortest reception path. The multipath signals received over other paths are thus usually delayed with respect to the reference signal and appear as trailing ghost images. It is possible, however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. When the receiver synchronizes to a reflected (longer path) signal, there will be a leading ghost image caused by the direct signal, or there will be a plurality of leading ghosts caused by the direct signal and other reflected signals of lesser delay than the reflected signal to which the receiver synchronizes. The parameters of the multipath signals--namely, the number of different-path responses, the relative amplitudes of the different-path responses, and the differential delay times between different ones of the different-path responses--vary from location to location and from channel to channel at a given location. These parameters may also be time-varying.
The visual effects of multipath distortion can be broadly classified in two categories: multiple images and distortion of the frequency response characteristic of the channel. Both effects occur due to the time and amplitude variations among the multipath signals arriving at the reception site. When the relative delays of the multipath signals with respect to the reference signal are sufficiently large, the visual effect is observed as multiple copies of the same image on the television display displaced horizontally from each other. These copies are sometimes referred to as "macroghosts" to distinguish them from "microghosts", which will be presently described. In the usual case in which the direct signal predominates and the receiver is synchronized to the direct signal, the ghost images are displaced to the right at varying position, intensity and polarity. These are known as trailing ghosts or "post-ghost" images. In the less frequently encountered case where the receiver synchronizes to a reflected signal, there will be one or more ghost images displaced to the left of the reference image. These are known as leading ghosts or "pre-ghost" images.
Multipath signals of relatively short delays with respect to the reference signal do not cause separately discernible copies of the predominant image, but do introduce distortion into the frequency response characteristic of the channel. The visual effect in this case is observed as increased or decreased sharpness of the image and in some cases loss of some image information. These short-delay, close-in or nearby ghosts are commonly caused by unterminated or incorrectly terminated radio-frequency transmission lines such as antenna lead-ins or cable television drop cables. In a cable television environment, it is possible to have multiple close-in ghosts caused by the reflections introduced by having several improperly terminated drop cables of varying lengths. Such multiple close-in ghosts are frequently referred to as "micro-ghosts".
Long multipath effects, or macroghosts, are typically reduced by cancelation schemes. Short multipath effects, or microghosts, are typically alleviated by waveform equalization, generally by peaking and/or group-delay compensation of the video frequency response.
Since the characteristics of a transmitted television signal are known a priori, it is possible, at least in theory, to utilize such characteristics in a system of ghost signal detection and cancelation. Nevertheless, various problems limit this approach. Instead, it has been found desirable to transmit repeatedly a reference signal situated, for example, in a section of the TV signal that is currently unused for video purposes and to utilize this reference signal for detection of ghost signals prior to arranging for the suppression of ghost signals. Typically, lines in the vertical blanking interval (VBI) are utilized. Such a signal is herein referred to as a Ghost Canceling Reference (GCR) signal; and a variety of different GCR signals have been described in patents and other technical publications.
Bessel pulse chirp signals are used in the GCR signal recommended for adoption as a standard for television broadcasting in the United States of America. The distribution of energy in the Bessel pulse chirp signal has a flat frequency spectrum extending continuously across the video frequency band. The chirp starts at the lowest frequency and sweeps upward in frequency therefrom to the 4.1 MHz highest frequency. The chirps are inserted into the first halves of selected VBI lines, the 19.sup.th line of each field currently being preferred. The chirps, which are on +30 IRE pedestals, swing from -10 to +70 IRE and begin at a prescribed time after the trailing edges of the preceding horizontal synchronizing pulses. The chirp signals appear in an eight-field cycle in which the first, third, fifth and seventh fields have a polarity of color burst defined as being positive and the second, fourth, sixth and eighth fields have an opposite polarity of color burst defined as being negative. The initial lobe of a chirp signal ETP that appears in the first, third, sixth and eighth fields of an eight-field cycle swings upward from the +30 IRE pedestal to +70 IRE level. The initial lobe of a chirp signal ETR that appears in the second, fourth, fifth and seventh fields of the eight-field cycle swings downward from the +30 IRE pedestal to -10 IRE level and is the complement of the ETP chirp signal.
The strategy for eliminating ghosts in a television receiver relies on the transmitted GCR signal suffering the same multipath distortions as the rest of the television signal. Circuitry in the receiver can then examine the distorted GCR signal received and, with a priori knowledge of the distortion-free GCR signal, can configure an adaptive filter to cancel, or at least significantly attenuate, the multipath distortion. A GCR signal should not take up too much time in the VBI (preferably no more than one TV line), but should still contain sufficient information to permit circuitry in the receiver to analyze the multipath distortion and configure a compensating filter to cancel the distortion.
The GCR signals are used in the television receiver for calculating the adjustable weighting coefficients of a ghost-cancelation filter through which the composite video signal from the video detector is passed to supply a response in which ghosts are suppressed. The weighting coefficients of this ghost-cancelation filter are adjusted so it has a filter characteristic complementary to that of the transmission medium giving rise to the ghosts. The GCR signals can be further used for calculating the adjustable weighting coefficients of an equalization filter connected in cascade with the ghost-cancelation filter, for providing an essentially flat frequency spectrum response (or other preferred frequency spectrum response) over the complete reception path through the transmitter vestigial-sideband amplitude-modulator, the reception medium, the television receiver front-end and the cascaded ghost-cancelation and equalization filters.
W. Ciciora et alii in "A Tutorial on Ghost Canceling in Television Receivers", IEEE Transactions on Consumer Electronics, vol. CE-25, 2/79, pp. 9-43, indicates that a GCR signal may appropriately exhibit a (sin x)/x waveform. Such a waveform, suitably windowed, exhibits a relatively constant spectral energy density over a frequency band of interest. Ghost locations can then be determined so a filters can be configured for ghost signal cancelation to reduce the effects of long multipaths and for waveform equalization to reduce the effects of short multipaths.
In U.S. Pat. No. 4,897,725 issued 30 Jan. 1990 to Tanaka et alii and entitled "GHOST CANCELLING CIRCUIT" a transmitted reference or GCR signal is used that is substantially the proposed BTA (Japanese) GCR signal and that utilizes as the main reference or deghosting signal a (sin x)/x waveform. This (sin x)/x waveform as received together with ghosts thereof is Fourier transformed to provide a set of Fourier coefficients. The Fourier transform of the ghosted GCR signal is then processed with an available Fourier transform of an unimpaired GCR to compute the deghosting filter parameters, that is, tap gain information for both an infinite-impulse-response (IIR) deghosting filter and a finite-impulse-response (FIR) waveform equalization filter.
U.S. Pat. No. 4,896,213 issued 23 Jan. 1990 to Kobo et alii and entitled "GHOST CANCELLING REFERENCE SIGNAL TRANSMISSION/RECEPTION SYSTEM" discloses a system with a built-in ghost cancelling device for reducing or eliminating ghost components attributable to group-delay distortion and frequency-versus-amplitude characteristic distortion generated in a signal reception path. A digital signal composed of frame synchronizing signals, clock synchronizing signals, and data signals is generated and superposed on a television signal to be transmitted, during a VBI scan line thereof. At the receiving end, the digital signal is utilized as a ghosted GCR signal in an arrangement that correlates that signal with its known non-ghosted GCR signal to control adaptive filtering of the video signal to reduce the ghost phenomenon.
U.S. Pat. No. 4,864,403 issued 5 Sep. 1989 to Chao et alii and entitled "ADAPTIVE TELEVISION GHOST CANCELLATION SYSTEM INCLUDING FILTER CIRCUITRY WITH NON-INTEGER SAMPLE DELAY" describes the use of an IIR deghosting filter using interpolative techniques.
U.S. Pat. No. 4,864,403 issued 10 Sep. 1991 to Koo and entitled "METHOD AND APPARATUS FOR COMMUNICATION CHANNEL IDENTIFICATION AND SIGNAL RESTORATION" describes method and apparatus for calculating ghost-suppression-filter parameters in a television receiver.
U.S. Pat. No. 4,044,381 issued 23 Aug. 1977 to Shimano et alii and entitled "AUTOMATIC WAVEFORM EQUALIZING SYSTEM FOR TELEVISION RECEIVER" describes a waveform equalizer filter as may be used to suppress microghosts.
U.S. Pat. No. 5,032,916 issued 16 Jul. 1991 to Matsura et alii and entitled "METHOD OF DETECTING SIGNAL WAVEFORM DISTURBANCE IN RECEIVED TELEVISION SIGNAL" describes the pairwise combination of VBI intervals containing antiphase GCR signals and in-phase other reference signals, in order to suppress longer-delayed macroghosts.
Since the known ghost-cancelation schemes rely to a high degree on cancelation procedures, the time-base stability of the GCR signal in the received television signal is critical in order for the procedure of determining the weights for the ghost cancelation and equalizing filters by analyzing the GCR signal to work well. The theoretical validity of a ghost-cancelation procedure using weighted summation of differentially delayed video signals depends on the same signal with different delays having given rise to the ghosted signal. If the length of scan lines is different during the GCR signal reception than during other portions of the video signal, then the weights determined for generating ghost-free GCR signal by weighted summation of variously delayed GCR signals will not be appropriate for generating ghost-free video at other times by weighted summation of variously delayed video signals. In a television receiver with included display device and ghost cancelation circuitry, the problem of time-base stability of the detected video signals is not a problem when receiving off-the-air broadcast signals or when receiving such signals as relayed by cable broadcasting or community antenna systems.
However, a television receiver with included display device and ghost-cancelation circuitry often will not perform its ghost-cancelation procedures satisfactorily when the receiver receives radio frequency (r-f) signal (or composite video signal) from a home video cassette recorder (VCR) that has recorded a television signal containing ghosts. Home VCRs use helical scanning of the electromagnetic tape, with head switching taking place shortly before the vertical retrace interval. There is time-base instability in the video signal reproduced from the electromagnetic tape during playback, which time-base instability unfortunately under practical circumstances often persists throughout the vertical retrace interval and to some extent in the first few active lines of video signal, which are used to generate the topmost portion of the picture on the display device of the television receiver. The weighing coefficients calculated by a microcomputer in the TV receiver in response to an evaluation of the GCR signal occurring in one scan line during the vertical blanking interval will not be correct for the active video signal in later scan lines of the same field, or of subsequent fields, because the scan lines of active video do not have the same actual time duration as the scan line in which GCR signal occurs. Even the VBI scan lines in which the GCR signals are included may have different durations from field to field.
Good time-base stability is essential also in implementing the inventions claimed herein, where the 19.sup.th scan lines of several fields are differentially delayed thereafter to be linearly combined in order to separate a GCR signal component from accompanying horizontal sync pulse, front porch, back porch including color burst and +30 IRE GCR signal pedestal components. These accompanying components will not cancel out well if there be errors in the timing of the samples of the 19.sup.th scan lines when those lines are digitized to facilitate their being differentially delayed using temporary digital memory. Home VCRs generally are not capable of providing the requisite time-base stability for separating GCR signal this way. U.S. patent application Ser. No. 07/955,016 discloses that this problem is obviated by including ghost-cancelation circuitry after the video detector of the television receiver front end included in a home VCR.